Current imaging metrology targets such as AIM (advance imaging metrology) targets provide a one dimensional signal using periodic one dimensional symmetric target elements such as gratings. Current imaging metrology targets such as Box-in-Box (BiB), Blossom or AIMid targets provide a two dimensional signal using non-periodic structures that usually have two symmetry axes such as bars, rectangles or crosses. These two types of current imaging metrology targets are exemplified in FIGS. 1A and 1B.
FIGS. 1A and 1B schematically illustrate representatives of two generic types of metrology targets, according to the prior art. A first type, represented by an AIM target 91 in FIG. 1A, is used to derive one-dimensional projection signals 65 of respective periodic elements 61, 62, 71, 72, which are used to calculate one-dimensional overlays. Corresponding pairs of periodic elements (having the same direction, each pair in a different layer, e.g., 61, 71 in one measurement direction and 62, 72 in another measurement direction) are used to calculate one-dimensional overlays between the respective one-dimensional projection signals 65 at the corresponding direction. The overall (two-dimensional) overlay is calculated from the two independent one-dimensional overlays. The typical size of AIM target 91 is several tens of micrometers, e.g., 20-40 μm (side length). A second type, represented by an AIMid target 92 in FIG. 1B, is used to derive two-dimensional centers 64, 74 from rotations (marked by arrows) of target elements 63, 73, 66, 76 (having, in the illustrated example, a four-fold rotational symmetry) in the plane. The pairs of target elements (e.g., 63, 66 in one layer, 73, 76 in another layer) are used to calculate a two-dimensional overlay between respective centers 64, 74 of the target elements in each layer. The typical size of AIMid target 92 is 10 μm (side length). AIM and AIMid targets 91, 92 may be multilayered, i.e., including respective pairs over multiple layers, and be used to derive one- and two-dimensional overlays respectively in a similar manner. The terms previous layer and current layer are used to refer to the former being produced prior to and lying below the latter (numerals 61, 62, 63, 66 refer to structures at the previous layer, numerals 71, 72, 73, 76 refer to structures at the current layer). No or any number of layers may separate the previous and current layers, and the target designs may be made to include several “previous” and/or several “current” layers.